Abstract

Increasing occurrences of extreme weather events, such as the 2018 drought over northern Europe, are a concerning issue under global climate change. High resolution archives of natural hydroclimate proxies, such as rapidly accumulating sediments containing biogenic carbonates, offer the potential to investigate the frequency and mechanisms of such events in the past. Droughts alter the barium (Ba) concentration of near-continent seawater through the reduction in Ba input from terrestrial runoff, which in turn may be recorded as changes in the chemical composition (Ba/Ca) of foraminiferal calcium carbonates accumulating in sediments. However, so far the use Ba/Ca as a discharge indicator has been restricted to planktonic foraminifera, despite the high relative abundance of benthic species in coastal, shallow-water sites. Moreover, benthic foraminiferal Ba/Ca has mainly been used in open ocean records as a proxy for paleo-productivity. Here we report on a new geochemical dataset measured from living (CTG-labelled) benthic foraminiferal species to investigate the capability of benthic Ba/Ca to record changes in river runoff over a gradient of contrasting hydroclimatic conditions. Individual foraminifera (Bulimina marginata, Nonionellina labradorica) were analyzed by laser-ablation ICP-MS over a seasonal and spatial gradient within Gullmar Fjord, Swedish west coast during 2018–2019. The results are compared to an extensive meteorological and hydrological data set, as well as sediment and pore-water geochemistry. Benthic foraminiferal Ba/Ca correlates significantly to riverine runoff, however, the signals contain both spatial trends with distance to Ba-source, and species-specific influences such as micro-habitat preferences. We deduce that shallow-infaunal foraminifera are especially suitable as proxy for terrestrial Ba input and discuss the potential influence of water-column and pore-water Ba cycling. While distance to Ba-source, water depth, pore-water geochemistry, and species-specific effects need to be considered in interpreting the data, our results demonstrate confidence in the use of Ba/Ca of benthic foraminifera from near-continent records as proxy for past riverine discharge and to identify periods of drought.

Highlights

  • 30 Extreme weather events, such as drought and heatwaves, have a devastating impact globally (e.g., Sternberg, 2011)

  • Droughts alter the barium (Ba) concentration of near-continent seawater through the reduction in Ba input from terrestrial runoff, which in turn may be recorded as changes in the chemical composition (Ba/Ca) of foraminiferal calcium carbonates accumulating in sediments

  • We report on a new geochemical dataset measured from living (CTG-labelled) benthic foraminiferal species to investigate the capability of benthic Ba/Ca to record changes in river runoff over a gradient of contrasting hydroclimatic conditions

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Summary

Introduction

30 Extreme weather events, such as drought and heatwaves, have a devastating impact globally (e.g., Sternberg, 2011). The drought and heatwave of 2018 that affected Northern and Central Europe, including Scandinavia, was exceptional in its severeness (e.g., Peters et al, 2020), and was followed by a warm and wet year in 2019 (Swedish Meteorological and Hydrological Institute, SMHI). This contrast in precipitation between 2018 and 2019 presents a unique opportunity to study 60 the response of benthic foraminiferal trace-elemental concentrations to changes in riverine discharge. We test the hypothesis of benthic foraminiferal Ba/Ca as a tracer for riverine discharge, assess the potential of identifying drought events in sediment foraminiferal records, and evaluate the broader implications for the interpretation of benthic Ba/Ca in paleo-studies in coastal environments. Except for periods of water renewals, the fjord’s waters are stratified in layers of distinct salinity and renewal times (e.g., Arneborg, 2004; compare Fig. 1)

Material and methods
Geochemical analyses of benthic foraminifera Living (CTG-labelled) specimens of
Benthic foraminiferal trace-element data processing
Pore-water and sediment trace-element analyses
Meteorological and hydrological data
Statistical analyses
Results and discussion
Sediment evidence for Ba cycling in Gullmar Fjord
Ba and metal oxide cycling 335
415 Acknowledgements

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